A rigid air-filled cavity backed UWB spiral antenna for GPR applications is designed, simulated, and measured. The fabricated physically big planar spiral is accurately cut out of a thick aluminum sheet using an abrasive water jet machine. It is fed by a mechanically firm balun feed that avoids the weak interconnection between the spiral and the balun. The antenna operates from 750 MHz – 1.25 GHz and maintains better than 12dB return loss and greater than 6 dB peak gain over the range. The fabricated antenna also keeps an axial ratio of less than 4 dB over the operating frequency range.